1. Field of the Invention
This invention relates to a method and apparatus for determining the existence and location of a contact between an underground electric wire conduit and an underground utility pipeline.
2. Description of Related Art
When an underground electric wire conduit containing one or more electric line wires crosses an underground utility pipeline, such as a natural gas pipeline, there are several potential events which may occur that result in the release of gas and a potential incident and/or explosion. If the electric wire conduit is in physical contact with the underground utility pipeline, then a shorting situation can occur. This will usually happen when one of the electric line wires, which are typically copper with insulation, within the electric wire conduit shorts to the conduit, which is typically made of metal, but which may also be made of concrete or plastic. In the case of a metal conduit, when one of the copper wires shorts to the conduit, the conduit itself can carry the current and voltage because it is also a conductor. This can also be the case with concrete conduits as well because they can conduct electricity under certain conditions. If the metal conduit is in physical contact with the underground utility pipeline, which is typically steel or other metallic conductor, then the current can “flow” to the pipeline, which can act as a “ground”. When this happens, there is usually an arcing situation between the electric wire conduit and the pipeline which, if allowed to go unchecked, will most likely result in a hole being formed in both the conduit and the pipeline, thereby releasing gas from the pipeline from which an explosion may follow. It is, thus, highly desirable to be able to locate these contact points between the conduit and the pipeline before there is a short of the wires to the conduit leading to arcing, gas release and explosion.
Potential solutions to this problem include ground penetrating radar and electromagnetic or acoustic pipe locators. However, these techniques do not provide the accuracy in the depth direction to determine whether or not there is a physical contact between the underground electric wire conduit and the underground utility pipeline. In addition, with both of these techniques, the operator must be either directly above or very close to the point of contact.
Placing an electrical or electromagnetic signal on either the pipeline or the electric wire conduit and then using a receiver on the pipeline to detect a physical contact between the conduit and the pipeline may not be possible because the pipeline may have individual pipeline sections connected by electrically-insulating mechanical couplings. As a result, the pipelines may not be electrically continuous and can severely attenuate an electrical or electromagnetic signal down their entire length, in addition to which the electric wire conduit also may not be electrically continuous.